Carbon nanotubes are well ordered, hollow carbon fibers composed of hexagonal groups of carbon atoms. Carbon nanotubes have been produced in both the single-walled and multi-walled forms. Single-walled nanotubes (SWNTs) are preferred in many applications, however, as they can be stronger, more conductive, and provide a higher surface area to volume ratio than their multi-walled counterparts.
Carbon nanotubes have shown appositeness in many applications, for instance as high energy density electrodes in electromechanical actuators and electrochemical batteries, as probes in scanning probe microscopy, as strengthening agents in composite materials when combined with other forms of carbon such as graphite fibers or carbon black, as catalyst supports in chemical applications, and as components in conductive ink dispersions. Other possible applications for SWNTs involve embedding the materials in semiconductor or insulator materials to obtain high interface areas for utilization in, for example, photo-voltaics, sensors, electroluminescent devices, and charge storage devices such as capacitors.
Applications of SWNTs can generally be divided into semiconducting applications and metallic applications. Unfortunately, most, if not all, formation methods provide a mixture of both metallic and semiconducting SWNTs. As such, great interest is currently being shown in the development of methods that produce only one or the other type of SWNT as well as in the development of methods to separate the two types following formation. For example, U.S. Pat. No. 6,183,714 to Smalley, et al., which is incorporated herein by reference, describes a dual laser pulse method for making ropes of SWNTs that are predominantly metallic in nature.
Other methods have attempted to synthesize one type of SWNT over the other through recognition of a difference in diameter between the two types. For example, some methods have attempted to control formation type by sufficiently controlling the catalyst particle size from which the tube grows. Particle size controls the diameter of the formed tube and hence the chirality and electronic nature of the tube. Such methods have met with limited success however, primarily due to the fact that the diametric differences between metallic and semiconductive SWNTs are so small. For instance the difference in diameter between a (10,10) metallic SWNT and a (9,11) semiconducting SWNT is only 0.03 Å.
Due to the difficulties presented in attempting to form only one type of SWNT, researchers have instead begun focusing more on methods to separate the types of SWNTs following formation. What is needed in the art is an improved method for separating metallic SWNTs from semiconducting SWNTs.